MSO (multi-service operators) provide several services to end users through a fiber optic network, with the final connection to the user through a coaxial connection. The services provided by the MSO typically include broadcast analog video, and narrow cast digital services such as data, VoIP, subscription pay per view, and video on demand (VOD) services. The services are allocated to a portion of the approximately available 1 GHz RF transmission spectrum on each optical channel in the network.
In recent years wavelength division multiplexed (WDM) optical transmission systems have been increasingly deployed in optical networks to meet the increased demand for bandwidth by providing more than one optical channel over the same optical fiber. The WDM techniques include coarse wavelength division multiplexed (CWDM) and dense wavelength division multiplexed (DWDM) systems. Whether a system is considered to be CWDM or DWDM simply depends upon the optical frequency spacing of the channels utilized in the system.
FIG. 1 shows a simplified block diagram of a conventional WDM transmission arrangement 100, which includes a radio frequency (RF) splitter 102, lasers 1041, 1042, 1043 and 1044, a wavelength division multiplexer (WDM) 106 and a single optical transmission path 108. As illustrated in the figure, a broadcast signal is split by RF splitter 102 into a plurality of information-bearing signals S1, S2, S3 and S4, each of which are a copy of the broadcast signal and hence contain the same information. The information-bearing signals are applied to and modulated on lasers 1041, 1042, 1043 and 1044, respectively. Narrowcast signals containing other information, such as digital data, video on demand data, and VoIP data are also provided to and modulated on lasers 1041, 1042, 1043, and 1044. Lasers 1041, 1042, 1043 and 1044 generate data modulated optical channels at wavelengths λ1, λ2, λ3 and λ4, respectively, where λ4>λ3>λ2>λ1. WDM 106 receives the optical channels and combines them to form a WDM optical signal that is then forwarded onto single optical transmission path 108.
As illustrated in the figure, narrowcast signals may be RF frequency multiplexed with broadcast channels. The narrowcast signals are typically digital signals and are normally much lower in amplitude than broadcast video signals. The arrangement of sending the same broadcast signal and different narrowcast signals over multiple wavelengths is a means of providing more segmentation in an optical network. RF splitter 102 splits the broadcast signal among lasers 1041, 1042, 1043 and 1044. As shown, each of lasers 1041, 1042, 1043 and 1044 receives a different narrowcast signal. The wavelengths carrying the combined broadcast and individual narrow cast signals, λ1, λ2, λ3 and λ4, respectively, are optically multiplexed onto single optical transmission path 108.
Conventional transmission lasers are capable of providing more than sufficient launch power to transmit a signal from end to end of a metropolitan distribution system, where the fiber link lengths are on the order of 15-30 kilometers. However, when multiple optical signals are provided on the same fiber, Raman cross-talk is induced. This Raman cross-talk creates distortion in the received signals. This will be discussed in more detail below.
Although WDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems is limited by various factors such as chromatic dispersion and the fiber nonlinearity, which can cause pulse shape change in the case of baseband digital signals and distortions in the case of analog signals. These impairments degrade the quality of the optically transmitted information. Fiber nonlinearities, for example, can give rise to crosstalk between optical signals operating at different wavelengths. When crosstalk occurs, modulation components of one signal are superimposed on another signal at a different wavelength. If the level of crosstalk is sufficiently large it will corrupt the information being transmitted by the optical signals impacted by this impairment.
One common cause of crosstalk, in an optical fiber communication system with multiple wavelengths, is Raman scattering. This type of crosstalk is caused by stimulated Raman scattering (SRS) in silica fibers (and other materials) when a pump wave co-propagates with a signal wave through it. Stimulated Raman scattering is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons).
FIG. 2 is a schematic diagram of the stimulated Raman scattering process. In the figure, a Raman media 202 has a transmission side 204 and a reception side 206. Signal photon 208, signal photon 210, pump photon 212, pump photon 214 and pump photon 216 travel from transmission side 204, through Raman media 202 toward reception side 206. At point 218, pump photon 212 is scattered in Raman media 202. As a result of the scattering event, pump photon 212 is annihilated and a new signal photon 220 at the Stokes frequency is created along with an optical phonon 222 at the Stokes shift frequency. Both energy and momentum are conserved:ωpump=ωsignal+ωOp phonon and {right arrow over (k)}pump={right arrow over (k)}signal+{right arrow over (k)}Op phonon,where ωx is the frequency of x and kx is the associated wavevector of x and  is Planck's constant divided by 2π.
FIGS. 3A-3C illustrate how the transfer of energy from Raman gain gives rise to crosstalk. FIGS. 3A-3C are simplified illustrations that are useful in facilitating an understanding of Raman crosstalk between two optical channels or signals Si and Sj, where Sj is at a longer wavelength than Si. FIG. 3A shows the signal Si and FIG. 3B shows the signal Sj. For simplicity of illustration Sj is shown as a signal with constant amplitude (i.e., a continuous string of zeros or ones in the case of baseband digital modulation). As indicated in FIG. 3C, the pattern of signal Si (dashed line) is impressed on the signal Sj by the process of Raman scattering interactions. In other words, signal Sj now includes as one of its components the pattern of signal Si. Likewise, since signal Si is pumping the signal Sj, the pattern of signal Sj (had it been modulated) would be impressed upon the pump Si by the process of pump depletion.
In addition to the generation of unwanted crosstalk the SRS process can also lead to the generation of Raman-induced second order (CSO: composite second order) and third order (CTB: composite triple beat) distortions. These distortions occur as result of the nonlinear nature of the Raman amplification process which, in the undepleted regime, is exponential in form.
Further, the Raman-induced crosstalk and nonlinear distortions are more pronounced when the wavelengths are located near the zero (i.e., near-zero) dispersion wavelength of the optical transmission media through which the signals are co-propagating (i.e., the optical fiber). In the case of a near-zero dispersion system the optical pump and signal waves are propagating at nearly identical group velocities through the media. The zero dispersion wavelength of a transmission media refers to the wavelength at which an optical signal will have no change in (inverse) group velocity with respect to changes in its optical frequency. The zero dispersion wavelength differs for different transmission media. In this case, the relative positions of the waves with respect to one another will remain nearly fixed throughout the length of the transmission media. Thus, if the signals Si and Sj are at or near the zero dispersion wavelength, they will largely maintain their relative phase with respect to one another. Hence, with very little walk off occurring between the optical channels the Raman-induced crosstalk and distortions can build up along the fiber in a constructive manner. The dispersion will generally increase as the wavelength difference between the optical signal and the zero dispersion wavelength increases. If the signals Si and Sj are located at wavelengths far displaced from the zero dispersion wavelength, their relative phases will change as they propagate down the transmission path. The levels of Raman-induced crosstalk and distortions are much lower in the nonzero dispersion scenario because, as the signals walk away from one another, it becomes more difficult for the crosstalk and distortions to build up constructively along the fiber length.
With reference again to FIG. 1, Raman-induced crosstalk may occur among the optical channels λ1, λ2, λ3 and λ4. Raman interactions cause both crosstalk interference and signal distortions on each optical channel. Because the amplitude of broadcast video signals is much higher than that of narrowcast digital signal, Raman-induced crosstalk has more impact on analog video signals between optical channels than on narrowcast channels.
Broadband communication system operators are currently seeking an inexpensive and efficient method to distribute directed programming (digital narrow cast) along with the analog broadcast programming to their customer base. One proposed method to accomplish this “fiber deep” application is through the use of a course wavelength division multiplexed (CWDM) optical system using wavelengths around 1310 nm.
Several serious obstacles exist and must be overcome before a workable 1310 CWDM system could actually be put into service. These problems arise as a result of the characteristics of the typical single mode fiber that is commonly deployed in the field and the required modulation scheme for the fiber deep architecture, which jointly render the proposed 1310 nm CWDM system useless.
The very small walk-off near 1310 nm coupled with the larger Raman gain factors means that the co-propagating optical signals will be more likely to interact with one another resulting in generally higher noise levels and distortion on the signals due to Raman interactions.
Additionally, in the fiber deep system each laser is modulated with identical analog broadcast information. This in conjunction with the enhanced probability of Raman interactions in the fiber produces new CSO and CTB distortions, which destroy the quality of the analog broadcast channels.
The deleterious effects of the Raman interaction between the co-propagating optical signals can be completely eliminated by simply reducing the launch powers of the CWDM laser transmitters. Unfortunately, reducing the launch powers means that the received signals, at the far end of the fiber, will have an unacceptably small carrier to noise ratios (CNR). The TV picture quality would again suffer.
FIG. 4A illustrates a conventional communication system 400 having an optical transmitter 402, a length of optical fiber 404 and an optical receiver 406. Optical fiber 404 is disposed between optical transmitter 402 at a transmitter side 408 and optical receiver 406 at a receiver side 410.
FIG. 4B is a graphical representation of light transmission through optical fiber 404. In the figure, the y-axis conveys the amount of power transmitted by each of the co-propagating signals and the x-axis conveys a distance from transmitter side 408 to receiver side 410. Solid line 412 represents no signal being transmitted by transmitter 402. Solid line 414 represents the maximum signal power that transmitter 402 can provide. Dotted line 416 represents a transmission power limit, whereas dotted line 418 represents a reception power threshold.
As discussed above, the amplitude of each transmitted co-propagating signal that optical fiber 404 may transmit, wherein each corresponding received signal is within a predetermined acceptable CNR, is limited by Raman interactions. As indicated by solid line 414 in FIG. 4B, optical transmitter 402 may be capable of providing each co-propagating signal having power that is greater than the transmission power limit as indicated by dotted line 416. Nevertheless, co-propagating signals having power higher than the transmission power limit as indicated by dotted line 416 will have unacceptable CNR ratios when received by receiver 406. That is, such signals may not be sufficiently processed without an unacceptable amount of noise and/or errors.
Optical receiver 406 at receiver side 410 has a predetermined level of detection, wherein a received signal power must be above a predetermined threshold in order to detect the signal at a predetermined acceptable CNR. If a detected signal power is below the predetermined threshold, then the CNR will be unacceptable such that the signal will be too noisy to process. In FIG. 4B, this example a reception power threshold is indicated by dotted line 418.
Further, optical transmission characteristics that are inherent in the media of optical fiber 404 attenuate a transmitted signal as a function of length along the direction of propagation through optical fiber 404. Accordingly, each co-propagating transmitted signal at transmitter side 408 of optical fiber 404 having a transmission power at the transmission power limit indicated by dotted line 416 will attenuate as it propagates toward receiver side 410. In this example, the signal attenuation of one of the co-propagating signals is indicated by line 420. The slope of attenuation may vary in accordance with changes in material in optical fiber 404. Further, although in this example the attenuation in the transmitted signal is linear, in other examples, the attenuation may be non-linear.
In order for receiver 406 to detect a transmitted co-propagating signal, as discussed above, within the predetermined CNR, the received signal power must be no lower than the reception power threshold indicated by dotted line 418. Accordingly, with a predetermined: 1) transmission power at the transmission power limit indicated by dotted line 416; 2) signal attenuation as indicated by line 420, which is based on optical transmission characteristics that are inherent in the media of optical fiber 404; and 3) reception power threshold is indicated by dotted line 418, a maximum length lmax of optical fiber 404 may be determined. This maximum length lmax of optical fiber 404 is the maximum length of optical fiber 404 that may be disposed between optical transmitter 402 and optical receiver 406, such that the deleterious effects of the Raman interactions between co-propagating signals, which cause distortions and crosstalk, and the fiber attenuation leading to degraded CNR at the receiver are at acceptable levels so as to still maintain a suitable system performance.
In order to have an acceptable CNR value, in analog optical transmission system, the received optical power at the far end of the fiber must typically be 0 dBm (1 mw). In a 20 km link, for example, the fiber loss is about 7 dB. In a conventional system as discussed above with respect to FIGS. 4A and 4B, a CWDM laser launch power must then be 7 dBm (5 mw) so that 0 dBm arrives at the receiver. A 7 dBm launch power is high enough to induce the Raman-induced crosstalk and Raman CSO/CTB distortions. Therefore, a conventional system is unable to successfully transmit CWDM signals over a 20 km link.
It is desirable to have a system and method that may increase the conventional maximum length lmax of optical fiber used in CWDM communication systems, while maintaining the convention transmission power of the transmission signal.
It is further desirable to have a system and method that may maintain the convention maximum length lmax of optical fiber used in CWDM communication systems, while decreasing the conventional transmission power of the transmission signal.